This invention is directed to flexible composites formed by coating a flexible substrate with a polymer material. Flexible composites made according to this invention can be formed into belts, processed into pressure sensitive tapes, or converted into rolled goods. Conveyor belts made from such composites can be used, for example, in the food processing industry for dough pressing applications and for transporting food through cooking applications. Pressure sensitive tapes and converted fabrics made from such composites can be used, for example, in the packaging industry for processing plastic bags as a release material.
Fluoropolymers have been used to create non-stick, flexible composites for over 50 years for use in the food cooking industries and the textile industries. Fluoropolymers are desirable as they provide high temperature stability, low surface energies that provide non-stick surfaces, and good flexibility. Belts composed of such composites are used, for example, in food processing facilities, where the food is conveyed through an oven or series of ovens on a fluoropolymer/fiberglass belt.
Typically, these flexible composites are manufactured in multiple layers, with the initial layers being used to impregnate a flexible base substrate. These impregnation layers are typically applied in a coating operation that involves one or more passes through a coating tower, with the impregnation passes occurring until the substrate is sufficiently closed, or free of voids or porosity. Once closed, subsequent coating layers are applied to the impregnated substrate to build film thickness on top of the flexible substrate and to build smoother surfaces with enhanced non-stick properties. Multiple thin layers, from multiple passes through the coating operation, are generally required in order to prevent mudcracking the coating surface.
The use of non-stick coating systems for metallic cookware has been known for over twenty years. As with flexible substrates, these coatings are applied in multiple layers consisting, typically, of a primer and a topcoat, but also can incorporate one or more midcoats. The primers used in these systems typically contain a heat resistant thermoplastic plastic binder, one or more fluoropolymer resins, pigments and fillers. In the primer, the thermoplastic and fluoropolymer are attached to each other via a mechanical bond, while the midcoats and/or topcoats are attached to one another via the fusing of fluoropolymer resins from each layer. An early such system is found in U.S. Pat. No. 4,049,863 to Vassiliou.
In the development of both flexible substrate and metallic cookware coatings, it is generally known that hard fillers can be used to increase the abrasion resistance and significantly reduce the cold-flow of the fluoropolymers. For examples see U.S. Pat. No. 4,049,863, U.S. Pat. No. 5,250,356 to Batzer, and U.S. Pat. No. 5,562,991 to Tannenbaurm. Typically, these hard fillers are inorganic and consist of hard metals, metal ceramics, ceramics, mica, and/or carbon-based materials. These materials continue to be the hard fillers generally used for both flexible substrate and cookware coatings.
Although significant research and use of thermoplastics have been prevalent in the cookware industry, heat resistant thermoplastics have not been widely used in flexible substrate coatings.
Flexible substrate based fluoropolymer materials tend to have a much shorter life than their cookware counterparts. This shorter lifecycle can be attributed significantly to the fragile nature of the substrate being coated. In cookware, the substrate of choice is typically aluminum. In flexible composites, it is typically woven fiberglass fabric. Fiberglass/fluoropolymer flexible composites typically have two methods of failure: mechanical failure and coating failure. Mechanical failure occurs when the fabric is torn or punctured, and is typically caused by human or equipment error or substrate degradation. Coating failure occurs when the substance contacting the fluoropolymer surface begins to stick, and is typically caused by the wearing or cracking of the fluoropolymer coating.
This faster rate of coating failure of flexible composite generally occurs because the fragile nature of the substrate prevents the addition of hard fillers at the high loading levels seen cookware coatings. Flexible composite fillers must be added in the midcoats and typically can only be added up to 10% by weight, while in cookware, hard filler loadings can be applied directly to the substrate and can have loading levels as high as 35% by weight. These hard fillers limit the cold-flow of the fluoropolymer coating and strengthen the fluoropolymer matrix.
Further, because of the fragile nature of fiberglass substrates, certain precautions typically must be taken in order to prevent the loss of tensile strength, tear strength, flexibility, and adhesion to the fabric. Typically, flexible substrates must be first impregnated (initially coated) with pure fluoropolymer resins in order to lubricate and protect the fiberglass filaments and yarns from abrading against each other. Although lubricating materials such as silicone fluids, waxes, and fluorosilicone-based materials can be incorporated into the initial impregnation coating(s), the majority of the coating, typically over 95%, is often fluoropolymer resin in order to maintain adhesion to the fiberglass.
It is generally known that the inclusion of fillers and pigments in the impregnation coating layers has a detrimental effect on the fiberglass tear strength and adhesion. U.S. Pat. Nos. 4,610,918 and 4,654,235 to Effenberger and U.S. Patent Application Publication 2002/0123282 to McCarthy disclose that woven fabrics are to be initially coated with fluoropolymer materials before the addition of fillers. To be effective, filler materials must be introduced in the overcoat layers of the coating, once the material has been fully impregnated (closed) by the fluoropolymer coating. If introduced before the flexible substrate has been closed, the fillers, which are typically abrasive materials, contact the glass surface and begin to degrade the properties of the substrate.
Although the use of non-fluorinated, heat resistant plastics (e.g., thermoplastics and thermosets) are well known to the cookware industry, these materials have had limited use in the coated flexible substrate industry. Although their use as fiberglass coatings has been disclosed in U.S. Pat. No. 6,846,570 to Leech, the coating, as disclosed, would not be viable due to the inclusion of high levels of ceramic fillers and pigments during the impregnation layers of the coating. As described by U.S. Pat. No. 6,846,570, the hard ceramics and pigments would create inter-filament abrasion that prevents the lateral movement of the glass filaments in a yarn and will result in substantially reduced tear strength and adhesion. Reduced tear and adhesion is a well-known and documented effect of these hard, abrasive materials against fiberglass. To be flexible and tear resistant, each filament in a fiberglass yarn must be free to move laterally in order to absorb the stresses of flexing. If this free movement is inhibited in any way, or if an abrasive material is incorporated in between filaments, the material significantly looses tear strength.
Due to the fragile nature of flexible substrates, the use of heat resistant plastics and thermosets has not been prevalent in the flexible composite industry. In fact, the use of such materials, to date, has been as a binder in the “overcoat” (midcoat) layers of the coating, after the substrate has been impregnated with fluoropolymer, and not as a primer layer, as in the cookware industry. U.S. Pat. Nos. 4,610,918 and 4,654,235 to Effenberger and U.S. Patent Application 2002/0123282 by McCarthy et al., disclose the use of thermoplastic/thermoset additives in the overcoat (i.e., midcoats) layers of non-stick, flexible substrate coatings. The “overcoat” layers, as disclosed by Effenberger are the layers of coatings applied after the woven substrate has been initially coated (impregnated) with fluoropolymer resins. Further, Effenberger states that these impregnation coatings are applied in two passes, minimally, and that these initial coatings are needed in order to minimize the stiffness of the composite and to facilitate adhesion to the substrate. McCarthy states that the initial coats constitute passes 1-3 through the coating oven and discloses that incorporating a high modulus thermoplastic or filler into the base pass on a woven fiberglass substrate may lead to a brittle product (low tear strength).
There is a need for a composite material that can be applied directly to flexible substrates and provide a hard, yet flexible, coating that will protect the flexible substrate from puncture, tear, and abrasion. There is also a need for an additive that, when added to fluoropolymer materials, prevents the fluoropolymer from cold-flowing and thus holds the coating in place even during periods of intense pressure on the substrate. There is further a need for a hard, flexible coating that would protect the flexible substrate and enable the application of overcoats that contain significantly higher percentage of fillers, thus enabling the coating to better withstand wear and abrasion. There is also a need for a flexible, conformable composite that retains its properties after folding and creasing. There is still yet a further need for a cost-effective, flexible composite material that possesses excellent dielectric properties and can be utilized in the manufacture of flexible circuitry.